This invention relates to an S/N (signal to noise) ratio determining apparatus for a receiver.
In all kinds of receivers, the S/N ratio of the received signal comes almost necessarily into question. As an example, a Loran-C receiver will be described. Loran-C, a well-known hyperbolic radio navigation technique, includes a chain of a single master station and two secondary stations. As shown in FIG. 1(a) of the accompanying drawings, the master station transmits a signal M of 9 Loran pulses during a group period Tp while the secondary stations transmit signals S.sub.1, S.sub.2 each including 8 pulses having the same period as signal M but with different predetermined time lags with respect to the transmission timing of signal M.
The Loran-C receiver receives the signals, as shown in FIG. 1(a), produces sample pulses Pa, Pb, Pc, (as shown in FIG. 1(b)), simultaneously with the third cycles of the first pulses of signals M, S.sub.1 and S.sub.2, and derives the current position of the receiver on the basis of time lags T.sub.1, T.sub.2 of sample pulses Pb, Pc relative to sample pulse Pa. Each of sample pulses Pa, Pb, Pc consists of a pair of subpulses P.sub.1, P.sub.2 which are spaced by one-quarter wavelength (2.5.mu. sec), as shown in FIG. 2(b), of the carrier wave Ca (100 KHz) of Loran pulse LP. The Loran receiver matches one of subpulses P.sub.1, P.sub.2 with the peak of the third cycle of carrier wave Ca after matching the other subpulses with the preceding zero-crossing point of carrier wave Ca, as shown in FIGS. 2(a) and 2(b). Such a Loran-C receiver is disclosed in examined Japanese patent publication 56-2312 published on Jan. 19, 1981.
In the above Loran-C receiver, when the received amplitude of Loran pulse LP is low, as shown in FIG. 2(a), noise Nz causes the amplitude of the carrier wave Ca to fluctuate significantly, so that the tracking points of the sample pulses fluctuate, thereby degrading measurement accuracy.
The receiver samples the received signal a plurality of times and derives the mean value of the sampled values to track the third cycle of carrier wave Ca. The number of samples processed by the receiver must be increased when the S/N ratio is low however, increasing in the number of processed samples prolongs the time required for measurement. Thus, a minimum requisite number of samples should be determined in accordance with the S/N ratio in order to minimize the time required for accurate measurement.
In order to satisfy this requirement, accurate measurement of the S/N ratio of the received signal is required.
One conventional method of measuring the S/N ratio involves separating the signal component from the noise component and deriving the ratio between the amplitudes of the signal and noise components. However, this method does not provide an accurate S/N ratio when the noise frequency is close to the signal frequency.
It could be contemplated that, with the Loran-C signal as described above, one more sample pulse P.sub.3 be produced at a point offset from Loran pulse LP to sample only noise Nz, as shown FIGS. 2(a) and (b), thereby deriving the ratio between the levels of the Loran pulse LP and noise Nz. However, in this case, noise Nz is also superimposed on the carrier wave in Loran pulse LP, so that the derived ratio would not be completely accurate. In particular, if the amplitude of noise Nz is not negligible compared to the amplitude of Loran pulse LP (i.e., when the S/N ratio is below about 10 dB), the accuracy of measurement of the S/N ratio is greatly affected.